1. Technical Field
The present invention is directed to electroluminescent devices with thin layers of organic material interposed between the anode and the cathode.
2. Art Background
Electroluminescent (EL) devices which have organic thin layers interposed between the anode and the cathode are used to fabricate flat panel displays. These EL devices are made into large-area, extremely thin full color displays. The structure of these devices is simple, as illustrated in FIG. 1. The EL device depicted in FIG. 1 consists of two layers of organic material, 10 and 12, interposed between an anode 14 and a cathode 16. The structure is formed on a glass substrate 18. In such devices, one of the organic layers functions as either a hole transporter or an electron transporter, and the other layer functions as an emitter layer. Typically, if the transporter layer is adjacent to the cathode, it functions as an electron transporter and if the transporter layer is adjacent to the anode it functions as a hole transporter.
In addition to EL devices that emit various discrete colors, EL devices that emit white light (or other unsaturated color light) are also desired. EL devices that emit white light are typically devices which emit light throughout the visible spectrum of red, green, and blue.
A device that emits white light is described in Berggren, M., et al., "White light from an electroluminescent diode made from poly3(4-octylphenyl)-2,2'-bithiophene! and an oxadiazole derivative," Journal of Applied Physics, 76:11, pp. 7530-7534 (December 1994). In the Breggren device, depicted in FIG. 1, the cathode 16 is aluminum, the anode 14 is indium-tin-oxide (ITO) and the organic layers 10 and 12 are poly(3-(4-octylphenyl)-2,2'-bithiophene) (PTOPT) and 2-(4-biphenylyl)-5-(4-tertbutylphenyl)-1,3,4-oxadiazole(PBD), respectively. In the Breggren devices, the PTOPT layer emits red and the PBD layer emits blue. There is no layer that emits green in the Breggren devices. Breggren represents that the emission of green light in its devices results from an interaction between the PBD and PTOPT layers.
The problems associated with EL devices are the efficiency and brightness of the emission and the stability of the organic films used to make the devices. Promising materials for EL devices are described in Tang, C. W., et al., "Electroluminescence of doped organic thin films," J.Appl.Phys., 65:9 pp. 3610-3616 (May 1, 1989). Tang et al. describe devices in which a layer of bis(triphenyl)diamine (TBD) and a layer of tris(8-hydroxyquinoline)aluminum (ALQ) are sandwiched between an ITO electrode and a Mg:Ag (1:10) electrode. The ALQ layer is reported to be two layers of ALQ with a layer of ALQ doped with either coumarin 540, DCM1, or DCM2 between the two ALQ layers. Tang et al. report that these devices emit in wavelength ranges from 450 nm to 675 nm (blue-green to orange-red). Tang et al. report that this wavelength range is imposed by the ALQ host.
Because of the efficiency of these emitting materials, devices that emit white light that use ALQ in conjunction with a diamine such as a bis(triphenyl)diamine derivative (TAD) are desired. However, such devices require additional materials to broaden the emission range to emit white light, particularly on the short wavelength side. These additional materials not only must emit in wavelengths which complement the emission spectrum of the ALQ, they should emit in that wavelength in the solid (i.e. film) state. Consequently, materials that, when incorporated into an EL device with an ALQ emitter layer, provide an EL device that emits white light, are desired.